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III.11.3 The Request Processor

By Pete Su

OpenACS docs are written by the named authors, and may be edited by OpenACS documentation staff.

This document is a brief introduction to the OpenACS 5.2.3rc1 Request Processor; more details can be found in the OpenACS 4 Request Processor Design. Here we cover the high level concepts behind the system, and implications and usage for the application developer.

The 5.2.3rc1 Request Processor is a global filter and set of Tcl procs that respond to every incoming URL reaching the server. The following diagram summarizes the stages of the request processor assuming a URL request like http://someserver.com/notes/somepage.adp.

Stage 1: Search Site Map

The first thing the RP does is to map the given URL to the appropriate physical directory in the filesystem, from which to serve content. We do this by searching the site map data model (touched on in the Packages, and further discussed in Writing OpenACS Application Pages). This data model maps URLs to objects representing content, and these objects are typically package instances.

After looking up the appropriate object, the RP stores the URL, the ID of the object it found, and the package and package instance the object belongs to into the environment of the connection. This environment can be queried using the ad_conn procedure, which is described in detail in OpenACS 4 Request Processor Design. The page development tutorial shows you how to use this interface to make your pages aware of which instance was requested.

Stage 2: Authentication

Next, the Request Processor examines the request for session information. Session information is generally sent from the client (the user's browser) to the server via cookies. The security/session handler is described in detail in its own document. It examines the client request and either extracts or sets up new session tokens for the user.

Stage 3: Authorization

Next, the Request Processor checks if the user has appropriate access privileges to the requested part of the site. In OpenACS 5.2.3rc1, access control is dictated by the permissions system. In this case, the RP checks if the user has "read" priviledges on the object in the site map specified by the URL. This object is typically a package instance, but it could easily be something more granular, such as whehter the user can view a particular piece of content within a package instance. This automatic check makes it easy to set up sites with areas that are only accessible to specific groups of users.

Stage 4: URL Processing, File Search

Finally, the Request Processor finds the file we intend to serve, searching the filesystem to locate the actual file that corresponds to an abstract URL. It searches for files with predefined "magic" extensions, i.e. files that end with: .html, .tcl and .adp.

If the RP can't find any matching files with the expected extensions, it will look for virtual-url-handler files, or .vuh files. A .vuh file will be executed as if it were a Tcl file, but with the tail end of the URL removed. This allows the code in the .vuh file to act like a registered procedure for an entire subtree of the URL namespace. Thus a .vuh file can be thought of as a replacement for filters and registered procs, except that they integrate cleanly and correctly with the RP's URL mapping mechanisms. The details of how to use these files are described in OpenACS 4 Request Processor Design.

Once the appropriate file is found, it is either served directly if it's static content, or sent to the template system or the standard Tcl interpreter if it's a dynamic page.

Once the flow of control reaches a dynamic page, the Request Processor has populated the environment of the request with several pieces of useful information. The RP's environment is accessible through the ad_conn interface, and the following calls should be useful to you when developing dynamic pages:

[ad_conn user_id]

The ID of the user associated with this request. By convention this is zero if there is no user.

[ad_conn session_id]

The ID of the session associated with this request.

[ad_conn url]

The URL associated with the request.

[ad_conn urlv]

The URL associated with the request, represented as a list instead of a single string.

[ad_conn file]

The actual local filesystem path of the file that is being served.

[ad_conn object_url]

If the URL refers to a site map object, this is the URL to the root of the tree where the object is mounted.

[ad_conn package_url]

If the URL refers to a package instance, this is the URL to the root of the tree where the package is mounted.

[ad_conn extra_url]

If we found the URL in the site map, this is the tail of the URL following the part that matched a site map entry.

[ad_conn object_id]

If the URL refers to a site map object, this is the ID of that object.

[ad_conn package_id]

If the URL refers to a package instance, this is the ID of that package instance.

[ad_conn package_key]

If the URL refers to a package instance, this is the unique key name of the package.

[ad_conn path_info]

In a .vuh file, path_info is the trailing part of the URL not matched by the .vuh file.